1. Field of the Invention
The present invention relates to a process for coating a substrate with a one-component coating composition containing oxime- or lactam-blocked polyisocyanates in the presence of acids or melamine resins to provide a coated substrate with improved resistance to yellowing.
2. Description of the Prior Art
There is a need in the automotive industry for topcoats which can be applied over existing basecoats and which provide improved properties when compared to conventional thermoset melamine/acrylics. Recently, two-component polyurethane coatings have increasingly been used as clearcoats. These coatings possess excellent environmental etch resistance as well as excellent appearance, durability, hardness and flexibility.
The two-component polyurethane coating compositions suffer from one major disadvantage, i.e., they require two-component spray equipment as opposed to the thermoset melamine/acrylics which are applied using one-component equipment. Therefore, an additional capital expenditure is required to obtain the necessary spray equipment for applying the two-component polyurethane coating compositions.
One possibility for providing a one-component polyurethane coating composition would be to convert the existing two-component polyurethane coating compositions by blocking the isocyanate groups of the polyisocyanate component. In this regard oxime-blocking agents are the preferred blocking agent because they possess storage stability in combination with a low unblocking temperature.
One of the difficulties of oxime- or lactam-blocked polyisocyanates is that when they come into contact with melamine resins or acids, they exhibit increased yellowing when compared to similar coatings which are not in contact with or blended with melamine resins or acids. The amount of yellowing increases as the curing temperature increases. Accordingly, blocking agents which require higher deblocking temperatures may suffer from increased yellowing due to the higher deblocking temperature. Such contact may occur when coating compositions containing oxime- or lactam-blocked polyisocyanates are applied as clearcoats over acid-containing and/or melamine-resin containing, thermoplastic or thermoset basecoats; when they are applied wet-on-wet as unicoats over acid-containing and/or melamine resin-containing primers; or when the coating compositions contain oxime- or lactam-blocked polyisocyanates in combination with acids and/or melamine resins.
Accordingly, it is an object of the present invention to provide one-component polyurethane coating compositions which may be applied in the presence of acids and/or melamine resins to provide coatings with improved resistance to yellowing without a significant reduction in any of the other valuable properties associated with polyurethane coating compositions.
This object may be achieved in accordance with the present invention by the use of the blocked polyisocyanates described hereinafter.